It has long been the desire to produce an improved audio speaker, i.e., one that effectively reproduces the input waveform without distortion over a wide frequency range. In general, the acoustic speaker system includes a current-carrying conductor, most commonly a coil, that reacts to the flux of a permanent magnet in the motor by axially moving in response to the amount of current in the coil, i.e. the Lorentz force B·I. In general, as the coil moves it drives a diaphragm, which creates the sound as a vibration in the air.
Distortions in the reproduced waveform are created by a number of causes, of which non-linear force and frequency imbalance are large contributors. A major factor in causing the speaker to have non-linear force is the coil moving outside the flux of the magnetic circuit, thereby reducing the B field+interacting with the current in the coil. This reduces the force generated and thus creates movement inconsistent with the original waveform. This is exacerbated at the lowest frequencies, where large excursions become necessary to produce sound. Indeed, the displaced volume required for a given volume level scales as the inverse square of the frequency (Vd∝1/f^2), thus requiring a driver to excurse four times as much to reproduce a signal at half the frequency. Ideally, for any driver, the force would remain constant over the required excursion, and would do so to large displacements in units requiring large excursion. This has been difficult and expensive to achieve with existing art.
Likewise, the inductance of a coil of wire, which is proportional to the length of that coil, reduces the current of high frequency signals flowing through the coil because of the increasing impedance from this inductance. This causes an increasing loss of force at higher frequencies, which distorts the signal by removing the upper frequency components to an increasing degree, distorting both the shape of the waveform and the frequency response. In extreme cases, the structure of the speaker causes excursion of the coil to modulate its inductance by position, causing an additional intermodulation distortion between low and high frequencies. Ideally, lower inductance is better, and the modulation of that inductance with position should be minimal. This has been difficult and expensive to achieve with existing art.
Various attempts have been made to solve non-linear excursion problems by increasing the length of the coil far beyond the size of the magnetic flux field (commonly referred to as “overhung”), or the reverse having a short coil in a long flux field (“underhung”), thereby allowing the coil to remain in the main flux over larger excursions. However, the longer coil leads to numerous additional problems, including increased mass, inductance, and intermodulation, and the attendant problems as stated above, as well as physical problems such as reduced tolerance to production variation and coil clearance from the rear of the speaker. The short coil in a long gap necessitates much larger and more expensive motor structures and is a less-than-ideal solution. Neither solution completely eradicates non-linearity in the force due to various magnetic effects.
As such, in general, most loud speaker systems that produce broad band audio energy utilize a plurality of acoustic drivers mounted within a common enclosure, each driver optimized for operation over its own limited band of frequencies. Each driver is driven through a crossover network to direct electrical signals with limited frequency content to the appropriate driver. These systems, using multiple speakers, have achieved considerable acceptance in the market place; however these systems are relatively expensive. Many attempts have been made in the past to design a single driver having a flat response over a wide band of frequencies driven by the potential advantages of lower cost, smaller size and the like. This has proven to be a difficult task because of the inherent conflict between the theoretically ideal system required to produce low frequency sound and that required to produce high frequency sound. To produce good low frequency sound you must move a relatively large mass of air by driving a large diameter rigid piston through a relatively long stroke; higher frequency requires a smaller diameter rigid piston driver through a shorter stroke. The displaced volume required for a given volume level scales as the inverse square of the frequency (Vd∝1/f^2). The theoretical criteria regarding the generation of high and low frequency sounds are in direct conflict. High frequency requires that the piston be accelerated at a high rate, thus ideally requiring a near-zero mass piston driven by a short coil, while low frequency requires lower acceleration of a larger, higher-mass piston through larger oscillatory amplitudes with a longer coil.
Whereas prior attempts to resolve the conflicts have focused upon reducing the mass and/or altering the suspension system and/or fabrication and mounting of the core and/or dividing the coil in half, it has been found by the inventors that utilizing what hereinafter will be called a “split gap design”, wherein a groove or series of grooves is placed in the exterior portion of the core and a similar groove or series of grooves is placed in the interior surface of the plate, allows a much shorter coil to accomplish the same purpose with little or no modification to the remainder of the speaker structure.
References known to the inventor include:
U.S. Pat. No. 2,004,735, granted to Thomas Jun. 11, 1935, which discloses improvements to dynamic loudspeakers, including the use of an actively-energized coil to neutralize changes in the gap flux density caused by variations in the field of the voice coil.
U.S. Pat. No. 3,983,337, granted to Babb Sep. 28, 1976, which discloses a plurality of changes to improve the performance of a broad band acoustics speaker, including the use of a pair of spaced coils that are used to modulate distortions by increasing the time that the coils are within the flux.
U.S. Pat. No. 4,188,711, granted to Babb Feb. 19, 1980, discloses a novel suspension system for use in a dynamic loud speaker.
U.S. Pat. No. 4,225,756, granted to Babb Sep. 30, 1980 discloses the methods of fabricating a speaker coil structure, including a rigid adhesive coating that transmits high frequency.
U.S. Pat. No. 4,661,973, granted to Takahashi Apr. 28, 1987, discloses a utilization of a tapered surface on the pole of the yoke or separate tapered plates attached to the annular plate.
U.S. Pat. No. 4,914,707, granted to Kato et al Apr. 3, 1990, discloses a pair of separate plates between which is mounted a magnet, wherein said annular magnet is recessed from the inner surface of the plates which interact with a pair of spaced coils permeated by magnetic fields of opposite polarity.
U.S. Pat. No. 5,151,943, granted to Van Gelder Sep. 29, 1992, discloses an improved output power for a dynamic loud speaker by decreasing the second harmonic distortion through the introduction of nonferromagnetic shielding members.
U.S. Pat. No. 5,202,595, granted to Sim et al Apr. 13, 1993, discloses a voice coil motor which comprises a yoke member and a central portion forming magnetic path left/right fringes and upper/lower fringes, and moving coil member around the central portion of the yoke member the permanent yoke magnets being adhered to the upper/lower fringes of the yoke member, and the yoke members being formed by overlapping at least two members of different permeabilities so as to make uniform the reluctance of lines of magnetic force being generated from the permanent magnets and flowing through the yoke member.
U.S. Pat. No. 5,550,332, granted to Sakamoto Aug. 27, 1996, discloses a loud speaker assembly for low frequency reproduction, wherein two magnets magnetizing in the direction of thickness has magnetic poles of the same polarity disposed facing each other with a center plate made of a soft magnetic material interposed therebetween.
U.S. Pat. No. 5,604,816, granted to Totani Feb. 18, 1997, discloses a vibrator for a speaker system wherein the coil is inserted into the gap and the magnetic pole is supported to the casing by rubber, elastic bodies.
U.S. Pat. No. 5,748,760, granted to Button May 5, 1998, discloses an improved electromagnetic transducer, combining a properly designed housing, a neodymium magnet and a dual coil structure also permeated by magnetic fields of opposite polarity.
U.S. Pat. No. 5,740,265, granted to Shirakawa Apr. 14, 1998, discloses a loud speaker unit, including a magnetic system of dual magnetic gaps formed with a permanent magnet creating magnetic fields of opposite polarity.